FIG. 1 illustrates a heat recovery steam generator (HRSG), which is used to turn otherwise wasted hot gasses into useful steam. Hot gasses enter 2 the HRSG from sources such as a gas turbine (not shown). In a HRSG, the hot gasses pass over heat transfer surfaces made up of tubes, usually with fins, in which water, under pressure is converted to steam. The heat from the hot gasses are transferred to either water, steam, or a combination of water and steam in a boiler tube, which is a type of heat exchanger. The steam rises in the tubes and is collected in a series of three drums, usually a high pressure (HP) drum 6, an intermediate pressure (IP) drum 8, and a low pressure (LP) drum 10. Ultimately, the hot gasses, after being depleted of most of their useful heat, are vented 4.
The internal workings of the HRSG in relation to the LP drum are shown in a schematic in FIG. 2. As is the convention, thick lines represent the passage of steam and thin lines the flow of water. Within the HRSG the LP drum 10 is used as the source of water for the boiler feedpump 15, which provides water, and eventually steam, to the HP and IP turbines 16. Eventually all steam, from the HP turbine, the IP turbine, the LP drum 10 and other sources goes to the LP turbine 18, where the last of the heat energy in the steam is changed into mechanical (rotary) energy. The remaining steam is passed to the condenser 20 where it is converted back into water, referred to herein as recycled condensate.
In the condenser steam passes over cooled tubes and condenses. The condenser is constructed so that non-condensable (volatile) materials are concentrated and removed by a vacuum pump as air exhaust 22. This process removes non-ionic impurities reasonably effectively, but ionic impurities, such as carbonic acid remain in the water phase. Makeup feed water 21 is added to the condenser to replace losses. The bulk of the flow from the condenser is steam recycled as condensate.
Since the HRSG is a closed system, volatile impurities present in the feed water are transferred to the produced steam, and then to the turbines (or turbine bypasses), then eventually to the condenser 20. During normal operations, only a small volume of ionogenic gas leaves the condenser 20 through the air exhaust 22, leaving a large portion of the ionogenic impurities in the recycled condensate. The recycled condensate is then pumped back to the LP drum as recycled water and the process is repeated without ridding the system of the volatile impurities.
Types of volatile impurities include ammonia, carbonates, such as CO2, and other chemicals. Concentrations as small as 10 parts per billion (ppb, μg/kg) are considered high, while a concentration of 100 ppb may quickly initiate corrosion that eventually leads failure of the turbine or other parts of the steam system. The corrosivity of volatile components varies, but some are clearly corrosive. Others may be more important as they blind the chemistry monitors to more corrosive but less volatile chemicals.
In the prior art, the volatile impurities are removed from the system by mass venting steam from the drums. This needs to be performed at startup of the HRSG, since the drum needs to be of sufficient temperature, and the HRSG system cannot operate while the drum is mass venting. Furthermore, in order to vent the volatile impurities, a large quantity of steam needs to be vented which wastes considerable amounts of water and heat, and creates noise pollution. Depending on the temperature in the drum at the time of venting and the quantity of volatile impurities, the mass venting process can last for 20-40 minutes, and vents 150,000-250,000 lbs of steam per hour (68,000-114,000 kg/hr). Over the course of a year over six million pounds (2.7 million kg) of steam can be vented in to rid the HRSG of volatile impurities.
It is possible to use purified water to reduce the impurities present. However, this is extremely expensive, and it is usually cheaper to vent impurities than use purified feed water. Further, volatiles, such as carbon dioxide continue to enter the system regardless of the purification and continue to create the problems discussed.
What is needed is an apparatus and method that can rid a system of volatile impurities without wasting large amounts of water and heat. Further what is needed is an apparatus and method that can rid a system of volatile impurities continuously during operation.